The term vibration, as used in this specification, includes noise, sound and other small amplitude linear disturbances and phenomena.
It is well known how to reduce the sound in passenger cabins, PCT/GB89/00964 and PCT/GB89/02021 by the same inventor have described the use of active sound control to reduce the vibration inside a passenger cabin. Both of these applications are herein incorporated by reference. PCT/GB89/00964 describes combining the power amplifiers for driving the loudspeakers with the loudspeakers to save cable mass and to improve heat dissipation. It also describes mounting the microphones in the cabin, attached to the trim. However, many of the crucial features necessary to make an active control system work effectively in a passenger cabin were not known until the current invention was made.
PCT/GB89/02021 describes one form of control system structure that can be used for this application. All digital controllers for active sound control sample the residual sound which is a combination of the original sound and the anti-sound. This sampling process allows the analogue signals to be converted into digital representation for processing. The controller described therein uses a time base for the sampling processes which is synchronized to the rotation rate of the propeller that is making the vibration to be controlled. Using this time base has advantages for the controller as the frequencies of the vibration are directly related to the sampling rate. This makes the transformation of the information into the frequency domain simple. However, if the frequencies change significantly due to the change in the rotation rate of the propellers the model of the system (embodied in the measured transfer functions) must change. The constant need for the model to change with the sampling frequency is a disadvantage. There is also a disadvantage for controlling multiple propellers at widely varying frequencies. As multiple sample rates or interpolation between sample rates must be used.